fRsi - the temperature factor, written fRsi - is a number between 0 and 1 that describes how warm the coldest inside surface of a junction stays. Think of it as the share of the way from the cold outside temperature up to the warm inside air temperature that the surface actually holds. A surface sitting right at inside-air temperature would score 1.0; one as cold as the outside would score 0. The higher the fRsi, the warmer that surface - and the less likely it is to reach dew point, sweat, and grow mould.
How fRsi is worked out
It's a steady-state figure, defined in ISO 13788: fRsi = (surface temperature − outside temperature) ÷ (inside air temperature − outside temperature). You don't calculate it by hand on a real junction - a thermal model does it, because the coldest spot usually hides at a corner, an edge or a frame-to-wall interface where heat finds a shortcut out. That's exactly the kind of cold spot fRsi is built to catch.
What does an fRsi of 0.75 mean?
0.75 is the number most people quote - it's the long-standing BRE IP1/06 screening benchmark for dwellings. A junction sitting at or above about 0.75 is treated as low risk; one below 0.75 is flagged as a condensation and mould risk worth a closer look. So when a report says "fRsi 0.75", it's pointing at that screening line: above it, probably fine; below it, investigate.
What's a good fRsi value?
Higher is better, because a higher number means a warmer inside surface. For dwellings the common screening target is around 0.75; some references use different thresholds for other occupancies or risk levels. But treat any single number as a flag, not a verdict - fRsi tells you where to look, not whether you've passed.
fRsi is the smoke alarm, not the fire report. It flags the junction - it doesn't prove the building complies.
The catch: fRsi is a screen, not the compliance test
This is where fRsi gets over-asked to do a job it was never meant to. Under NCC 2025, the condensation provisions - Parts F8 and H4 - don't judge compliance on fRsi. They judge it on a Mould Index, which estimates how much mould would actually grow on a surface over time. fRsi says "this junction looks cold"; the Mould Index says "and here's whether that actually becomes a problem." The number that clears the box is the Mould Index - and a TIRS MI-6 assessment is what produces it, with the supporting science, on the junction that matters.
If you want the full picture of how these three terms fit together - the screen, the compliance measure, and the assessment - there's a longer piece on fRsi, the Mould Index and TIRS.
Got a junction that's screening below 0.75?
Tell us the element and the climate zone. We'll say plainly whether the fRsi screen settles it or whether you need a Mould Index (TIRS MI-6) for the NCC condensation evidence - before you spend on the wrong one.
Ask the lab →About fRsi. fRsi (the temperature factor, fRsi) is an ISO 13788 steady-state screening indicator only and does not certify compliance. The 0.75 threshold is the BRE IP1/06 screening benchmark for dwellings. Under NCC 2025, Parts F8 and H4 determine condensation compliance from a Mould Index; TIRS MI-6 delivers that Mould Index with its supporting science on the relevant element. Results are indicative to the systems, elements and conditions provided; final NCC compliance is confirmed by your assessor or certifier. AFRC Lab (Advanced Fenestration Research & Certification) is an independent consultancy; it is not the AFRC. TIRS™ is a trademark of AFRC Lab Pty Ltd.